1. Field of the Invention
The present invention relates to a numerical control device of a machine tool for performing tapping process on a workpiece.
2. Description of the Related Art
When using a machine tool for performing a tapping process on a workpiece, it is necessary to precisely synchronize the spindle to which the tapping tool is attached and the feed shaft for feed operation of the spindle. Further, to shorten the cycle time of the tapping process, it is necessary to determine the rotational speed and acceleration etc. of the spindle so that the movement time of the spindle by the feed shaft is minimized for a predetermined rotation amount of the spindle. However, accurate determination of these parameters largely depends on the ability and experience of skilled operators, so has been difficult task for most operators.
In relation to this, JP-A-H3-117515 discloses the art of calculating an acceleration/deceleration time constant T for minimizing the machining time, based on a cutting depth “l” and thread pitch P during tapping process. More specifically, the art of JP-A-H3-117515 calculates the acceleration/deceleration time constant T, using a relational expression: S=l/P=αT2 which is established among a rotation amount S(S=l/P) of the spindle, acceleration α of the spindle, and an acceleration/deceleration time constant T. However, the art of JP-A-H3-117515 is intended to calculate the suitable acceleration/deceleration time constant for a predetermined rotational speed of the spindle, and it cannot be used to determine the rotational speed of the spindle for minimizing the machining time. Further, the art of JP-A-H3-117515 envisions only the situation where a spindle starts to decelerate immediately after acceleration up to a command rotational speed, so cannot necessarily calculate the optimal acceleration/deceleration time constant for all situations.
A numerical control device which can determine the command rotational speed of the spindle for minimizing the machining time during tapping process has been sought.